This invention relates to wooden boats and in particular to a new method of hull construction using double, longitudinal, sealed, alternate and prestressed planking.
Traditional hull construction with wooden planking generally uses one of two building methods: single planking or double planking. Single planking is the more commonly used method of the two. In this method a single layer of individual planks are longitudinally placed and bent in a fore and aft direction across the ribs of a boat, the ribs defining the shape of the boat. The planks are fastened to the ribs with various mechanical fasteners such as nails, screws, rivets, etc. The longitudinal edges of the planks abut against each other, and are usually beveled to form a "V" groove seam that opens outboard, i.e., away from the center of the hull. The "V" groove seam is then filled snugly with various types of caulking material such as oakum, cotton, etc., and covered with a flexible seam compound. When the boat is placed in water, the planks swell thereby closing the seams. The caulking material provides watertightness between the planks and flexibility to compensate for expansion.
There are drawbacks with the single planking method. Over time the caulking medium tends to physically deteriorate and move within the seams. In addition, there is alternate shrinking and swelling of the planks as the boat is wetted and dried by the sea and sun, and also as the boat is hauled in and out of the water for storage, maintenance and repairs. The alternate shrinking and swelling cracks the hull sealer coats, i.e., paints and varnishes, which have been applied over the hull. The cracking is especially prevalent at the caulked longitudinal seams. This, plus the deterioration and movement of the caulking material, breaks the watertight integrity of the hull and breaks the caulking-to-wood bond within the seams, thus causing serious leaks. Leakage leads to rotting and weakening of the wooden planks and causes additional deformation by alternately wetting and drying the hull planking which in turn accelerates and compounds the cycle of deterioration. A further drawback with the single planking method, and possibly the most serious, is that the strength of a single planked hull is based on a single layer of wooden planks. Deterioration of the planks leads to direct loss of strength. This is further aggravated by the traditional method of building single plank boats which results in each plank behaving relatively independently under both normal and catastrophic stresses. Mechanical fasteners limit stress dispersal between individual planks and, indeed, often tend to focus stress upon themselves. Stresses, instead of being harmlessly dispersed throughout the entire structure of the hull, tend to remain concentrated at the point where they occur, thus jeopardizing the hull.
The second traditional method of hull construction with wooden planking, double planking, is an attempt to overcome the strength limitations of the single planking method. In this form two layers of alternating, longitudinal wooden planks are fastened to the ribs. Essentially, a hull with two skins is built using two layers of planking, one layer of planking lying directly atop the other. The first or inboard, i.e., toward the center of the hull, layer of planks consists of individual wooden planks longitudinally positioned and bent in a fore and aft direction across the ribs of a boat. The planks are fastened to the ribs with mechanical fasteners as in the single planking method. The longitudinal edges of the planks are not beveled to form a caulking groove, but abut flat to each other, normally with no glue or sealer between them. The second or outboard layer is positioned longitudinally in a fore and aft direction over the first layer. The longitudinal edges of the outboard layer planks are generally aligned over the inboard layer planks so that they are positioned to lie approximately midway between the longitudinal edges of the inboard layer planks. The second layer of planks is then mechanically fastened through the first layer of planks to the ribs. The longitudinal edges of the outboard layer planks are beveled, as in the single planking method, to form a " V" groove seam that opens outboard. The "V" grooves are caulked and sealed as in the single planking method.
The first and second layers of planks in the double planking method are normally separated with a non-adhesive sealer, such as shellac, or with various kinds of rot preventers, such as a membrane of canvas covered with white lead. Neither of these methods prevent water seepage between the two layers of the hull, for many of the same reasons as in the single planking method. Trapped water, plus lack of ventilation, leads to wood rot. Despite its added strength, a double planked hull shares the major imperfections of a single planked hull, with the added problem that inspection for any deterioration between the two layers of planks is impossible without removing the outer planks. Therefore, rot between the inner and outer layers of planks is extremely difficult to locate and repair.
The relative independence and actual movement of each individual, mechanically fastened plank within a single planked or double planked hull is ultimately the major cause of hull deterioration and failure. Various methods have been used to minimize plank movement and independence. Hulls have been reinforced by inlaying into them diagonally running wooden or metal members. The prolific use of metal or wooden knees at critical structural junctures, to transfer stress, triangulate the planking-rib matrix, and tighten corners has also been tried. However, mechanical fastenings and the use of diagonal bracing and knees, address only the symptoms and not the primary problem of traditional wooden plank hull construction: independent, separately working wooden planks.